Hi all,
Apologies for the late reply to this - we've been considering the problem and I'd like to report back on what (we think) has fixed it in case anyone else is in a similar position.
We decided to take the seats out, strip the trim and take the carpet out to evaluate the condition of the underlay and, as we expected, the underlay was black in patches and carried the smell. The smell actually got worse the closer we got to the carpet, so we figured we were onto a winner. The underlay has been bagged and, when we're able to, we'll scrap this at the tip as there is no value in keeping it.
I've now gone in and vacuumed/cleaned the metal and refitted the seats so that it's driveable. The trim has been scrubbed clean and we've treated the underside of the carpet to a carpet cleaner and stored all of it in the garage until we can sort some new underlay out. The trim and carpet smell lovely and, so far, the smell in the car seems to have disappeared!
Now we just to sort some new underlay out (I have no idea where to buy this from!) and refit the carpet and seats for good, knowing the shell of the car is solid and clean.
So I just wanted to mention that we think we've solved it, and we'll let the metal shell breathe for a few weeks/months before fitting it all back together
Thanks for all your help/comments with this one.